Method for improved performance of a vehicle having an internal combustion engine

ABSTRACT

An engine control system for managing lean operation of an internal combustion engine coupled to a lean NOx trap uses an indication of vehicle activity. A target amount of tailpipe emission per distance is adjusted based on the vehicle activity indication. Then, engine air-fuel ratio is controlled based on the adjusted target amount of tailpipe emission per distance. Also, the indication of vehicle activity is used to enable decontamination cycles of the lean NOx trap.

This is a continuation of U.S. Ser. No. 09/528,505, filed 03/17/2000,now U.S. Pat. No. 6,477,832 having the same assignee, and which isincorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The invention relates to a system and method for controlling an internalcombustion engine coupled to an emission control device.

BACKGROUND OF THE INVENTION

In direct injection spark ignition engines, the engine operates at ornear wide-open throttle during stratified air-fuel ratio operation inwhich the combustion chambers contain stratified layers of differentair-fuel ratio mixtures. Strata closest to the spark plug contain astoichiometric mixture or a mixture slightly rich of stoichiometry, andsubsequent strata contain progressively leaner mixtures. The engine mayalso operate in a homogeneous mode of operation with a homogeneousmixture of air and fuel generated in the combustion chamber by earlyinjection of fuel into the combustion chamber during its intake stroke.Homogeneous operation may be either lean of stoichiometry, atstoichiometry, or rich of stoichiometry.

Direct injection engines are also coupled to emission control devicesknown as three-way catalytic converters optimized to reduce CO, HC, andNOx. When operating at air-fuel ratio mixtures lean of stoichiometry, athree way catalyst optimized for NOx storate, known as a NOx trap orcatalyst, is typically coupled downstream of the first three-waycatalytic converter.

During lean, rich, and stoichiometric operation, sulfur contained in thefuel can become trapped in the emission control device in the form ofSOx. This gradually degrades emission control device capacity forstoring NOx, as well as emission control device efficiency. Tocounteract sulfur effects, various sulfur decontamination methods areavailable.

One method determines to perform a decontamination cycle when leanengine operation occurs simultaneously with high exhaust gas or NOx traptemperature. Such a method is discloses in U.S. Pat. No. 5,402,641.

The inventors herein have recognized a disadvantage with the aboveapproach. In particular, these conditions do not provide the bestatmosphere for controlling sulfur contamination. In particular, theseconditions may occur during large transient conditions where the engineoperation is changing widely and quickly. Performing decontaminationusing known methods under such conditions results in less accuratetemperature control and less efficient decontamination. In particular,inaccurate temperature control may lead to degradation of the emissioncontrol device.

SUMMARY OF THE INVENTION

An object of the invention claimed herein is to provide a method forenabling emission control device decontamination cycles.

The above object is achieved, and disadvantages of prior approachesovercome, by a method for controlling an internal combustion engine of avehicle, the engine coupled to an emission control device susceptible toreversible contamination, the method comprising: generating anindication of vehicle activity based on an operating condition;determining a performance impact of performing a decontamination cycle,where said decontamination cycle reverses the reversible contamination;and operating the engine based on said indication and said performance.

By using an indication of vehicle activity, it is possible to performsulfur decontamination cycles under conditions where improvedtemperature control is possible. More accurate temperature control canlead to more efficient decontamination. In other words, transientdisturbances, which degrade temperature control, cause temperaturedeviation from a desired temperature. Performing decontamination cycleswhen such disturbances are unlikely reduces these deviations and therebyimprove control.

An advantage of the above aspect of the present invention is improvedfuel economy due to improved temperature control.

Another advantage of the above aspect of the present invention isimproved emission control device durability due to improved temperaturecontrol.

Other objects, features and advantages of the present invention will bereadily appreciated by the reader of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages described herein will be more fullyunderstood by reading an example of an embodiment in which the inventionis used to advantage, referred to herein as the Description of PreferredEmbodiment, with reference to the drawings, wherein:

FIGS. 1–2 are block diagrams of an embodiment wherein the invention isused to advantage; and

FIGS. 3–15 are high level flow charts of various operations performed bya portion of the embodiment shown in FIG. 1.

DESCRIPTION OF THE INVENTION

Direct injection spark ignited internal combustion engine 10, comprisinga plurality of combustion chambers, is controlled by electronic enginecontroller 12 as shown in FIG. 1. Combustion chamber 30 of engine 10includes combustion chamber walls 32 with piston 36 positioned thereinand connected to crankshaft 40. In this particular example, piston 30includes a recess or bowl (not shown) to help in forming stratifiedcharges of air and fuel. Combustion chamber 30 is shown communicatingwith intake manifold 44 and exhaust manifold 48 via respective intakevalves 52 a and 52 b (not shown), and exhaust valves 54 a and 54 b (notshown). Fuel injector 66 is shown directly coupled to combustion chamber30 for delivering liquid fuel directly therein in proportion to thepulse width of signal fpw received from controller 12 via conventionalelectronic driver 68. Fuel is delivered to fuel injector 66 by aconventional high pressure fuel system (not shown) including a fueltank, fuel pumps, and a fuel rail.

Intake manifold 44 is shown communicating with throttle body 58 viathrottle plate 62. In this particular example, throttle plate 62 iscoupled to electric motor 94 so that the position of throttle plate 62is controlled by controller 12 via electric motor 94. This configurationis commonly referred to as electronic throttle control (ETC) which isalso utilized during idle speed control. In an alternative embodiment(not shown), which is well known to those skilled in the art, a bypassair passageway is arranged in parallel with throttle plate 62 to controlinducted airflow during idle speed control via a throttle control valvepositioned within the air passageway.

Exhaust gas oxygen sensor 76 is shown coupled to exhaust manifold 48upstream of catalytic converter 70. In this particular example, sensor76 provides signal UEGO to controller 12 which converts signal UEGO intoa relative air-fuel ratio λ. Signal UEGO is used to advantage duringfeedback air-fuel ratio control in a manner to maintain average air-fuelratio at a desired air-fuel ratio as described later herein. In analternative embodiment, sensor 76 can provide signal EGO (not show)which indicates whether exhaust air-fuel ratio is either lean ofstoichiometry or rich of stoichiometry.

Conventional distributorless ignition system 88 provides ignition sparkto combustion chamber 30 via spark plug 92 in response to spark advancesignal SA from controller 12.

Controller 12 causes combustion chamber 30 to operate in either ahomogeneous air-fuel ratio mode or a stratified air-fuel ratio mode bycontrolling injection timing. In the stratified mode, controller 12activates fuel injector 66 during the engine compression stroke so thatfuel is sprayed directly into the bowl of piston 36. Stratified air-fuelratio layers are thereby formed. The strata closest to the spark plugcontains a stoichiometric mixture or a mixture slightly rich ofstoichiometry, and subsequent strata contain progressively leanermixtures. During the homogeneous mode, controller 12 activates fuelinjector 66 during the intake stroke so that a substantially homogeneousair-fuel ratio mixture is formed when ignition power is supplied tospark plug 92 by ignition system 88. Controller 12 controls the amountof fuel delivered by fuel injector 66 so that the homogeneous air-fuelratio mixture in chamber 30 can be selected to be substantially at (ornear) stoichiometry, a value rich of stoichiometry, or a value lean ofstoichiometry. Operation substantially at (or near) stoichiometry refersto conventional closed loop oscillatory control about stoichiometry. Thestratified air-fuel ratio mixture will always be at a value lean ofstoichiometry, the exact air-fuel ratio being a function of the amountof fuel delivered to combustion chamber 30. An additional split mode ofoperation wherein additional fuel is injected during the exhaust strokewhile operating in the stratified mode is available. An additional splitmode of operation wherein additional fuel is injected during the intakestroke while operating in the stratified mode is also available, where acombined homogeneous and split mode is available.

Nitrogen oxide (NOx) absorbent or trap 72 is shown positioned downstreamof catalytic converter 70. NOx trap 72 absorbs NOx when engine 10 isoperating lean of stoichiometry. The absorbed NOx is subsequentlyreacted with HC and catalyzed during a NOx purge cycle when controller12 causes engine 10 to operate in either a rich mode or a nearstoichiometric mode.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, anelectronic storage medium for executable programs and calibrationvalues, shown as read-only memory chip 106 in this particular example,random access memory 108, keep alive memory 110, and a conventional databus.

Controller 12 is shown receiving various signals from sensors coupled toengine 10, in addition to those signals previously discussed, including:measurement of inducted mass air flow (MAF) from mass air flow sensor100 coupled to throttle body 58; engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling sleeve 114; a profile ignitionpickup signal (PIP) from Hall effect sensor 118 coupled to crankshaft 40giving an indication of engine speed (RPM); throttle position TP fromthrottle position sensor 120; and absolute Manifold Pressure Signal MAPfrom sensor 122. Engine speed signal RPM is generated by controller 12from signal PIP in a conventional manner and manifold pressure signalMAP provides an indication of engine load.

In this particular example, temperature Tcat of catalytic converter 70and temperature Ttrp of NOx trap 72 are inferred from engine operationas disclosed in U.S. Pat. No. 5,414,994, the specification of which isincorporated herein by reference. In an alternate embodiment,temperature Tcat is provided by temperature sensor 124 and temperatureTtrp is provided by temperature sensor 126.

Fuel system 130 is coupled to intake manifold 44 via tube 132. Fuelvapors (not shown) generated in fuel system 130 pass through tube 132and are controlled via purge valve 134. Purge valve 134 receives controlsignal PRG from controller 12.

Exhaust sensor 140 is a sensor that produces two output signals. Firstoutput signal (SIGNAL1) and second output signal (SIGNAL2) are bothreceived by controller 12. Exhaust sensor 140 can be a sensor known tothose skilled in the art that is capable of indicating both exhaustair-fuel ratio and nitrogen oxide concentration.

In a preferred embodiment, SIGNAL1 indicates exhaust air-fuel ratio andSIGNAL2 indicates nitrogen oxide concentration. In this embodiment,sensor 140 has a first chamber (not shown) in which exhaust gas firstenters where a measurement of oxygen partial pressure is generated froma first pumping current. Also, in the first chamber, oxygen partialpressure of the exhaust gas is controlled to a predetermined level.Exhaust air-fuel ratio can then be indicated based on this first pumpingcurrent. Next, the exhaust gas enters a second chamber (not shown) whereNOx is decomposed and measured by a second pumping current using thepredetermined level. Nitrogen oxide concentration can then be indicatedbased on this second pumping current.

In one aspect of the present invention, a determination of degradationof the nitrogen oxide concentration measurement can be made if it isdetermined that the exhaust air-fuel ratio measurement is degraded. Thisis because nitrogen oxide concentration is not accurately detected inthe second chamber unless the first chamber controls oxygen partialpressure properly. In other words, if it is determined that operation ofthe first chamber (where partial pressure of oxygen is measured) isdegraded, then it is possible to determine that the second signal(SIGNAL2) indicating nitrogen oxide concentration is degraded asdescribed later herein with particular reference to FIG. 13.

Referring now to FIG. 2, a port fuel injection engine 11 is shown wherefuel is injected through injector 66 into intake manifold 44. Engine 11is operated homogeneously substantially at stoichiometry, rich ofstoichiometry, or lean of stoichiometry. Fuel is delivered to fuelinjector 66 by a conventional fuel system (not shown) including a fueltank, fuel pumps, and a fuel rail.

Those skilled in the art will recognize, in view of this disclosure,that the methods of the present invention can be used to advantage witheither port fuel injected or directly injected engines.

Referring now to FIGS. 3–5, routines for determining performance impactsof operating in various engine operating conditions are described. In apreferred embodiment, performance impact is a fuel economy percentageimpact over stoichiometric operation. The impact can be a benefit, wherefuel is saved over stoichiometric operation, or a fuel loss. In otherwords, the following routines determine fuel economy saved relative tostoichiometric operation or fuel economy lost relative to stoichiometricoperation. However, those skilled in the art will recognize in view ofthis disclosure various other performance impacts that can be used tocompare different operation modes such as, for example, fuel usageimpact, fuel efficiency impact, fuel savings, fuel loss, engineefficiency impact, fuel savings per distance traveled by the vehicle, ora driveability impact.

Referring now specifically to FIG. 3, a routine is described fordetermining a maximum fuel economy benefit that can be provided whenoperating lean, assuming that emission control device 72 has beendecontaminated. More specifically in a preferred embodiment, that asulfur decontamination has been completed. In other words, the routinedetermines the maximum potential fuel economy benefit that can beprovided after performing a decontamination cycle. First, in step 308,counter j is reset equal to zero. Next, in step 310, a determination ismade as to whether a decontamination cycle has just been completed. Adecontamination cycle, as described herein, refers to any operatingcycle where engine operating conditions are changed to remove acontaminant. For example, a sulfur decontamination cycle where exhaustgas temperature is raised and the engine is operated substantially at orrich of stoichiometry to remove sulfur contaminating emission controldevice 72 is one such decontamination cycle. When the answer to step 310is YES, the routine continues to step 312 where parameter OLD_FE_MAX isset equal to parameter FILTERED_FE_MAX. Also, in step 312, counter j isset equal to one. Counter j keeps track of the number of NOx fill/purgecycles after a decontamination cycle over which the maximum fuel economybenefit is average. Next, in step 314, a determination is made as towhether a NOx fill/purge cycle has just been completed. When the answerto step 314 is YES, a determination is made as to whether counter jequals one. When the answer to step 316 is YES, the routine continues tostep 318. In step 318, the routine calculates temporary fuel economybenefit (FE_TEMP_(j)) based on current fuel economy benefit (FE_CUR),where current fuel economy benefit is calculated as described below.This temporary fuel economy benefit represents the fuel economy benefitaveraged over a NOx fill/purge cycle that is achieved compared tooperating the engine substantially at stoichiometry. Next, in step 320,maximum fuel economy benefit (FE_MAX) is calculated based on temporaryfuel economy benefit. Next, in step 322, counter j is incremented. Next,in step 324, a determination is made as to whether counter j is greaterthan predetermined value J1. Predetermined value J1 represents thenumber of NOx fill/purge cycles after a decontamination cycle over whichmaximum fuel economy benefit, provided by lean operation relative tostoichiometric operation, is calculated. In a preferred embodiment,predetermined value J1 represents the number of NOx fill/purge cyclesafter a decontamination cycle over which maximum fuel economy benefit,is averaged. This averaging allows variations in vehicle operatingconditions to be accounted in determining maximum fuel economy benefitso that a representative value is obtained. When the answer to step 324is YES, the filtered maximum fuel economy benefit (FIL_FE_MAX) is setequal to maximum fuel economy benefit.

Continuing with FIG. 3, when the answer to step 316 is NO, temporaryfuel economy benefit (FE_TEMP_(j)) is calculated in step 326 based oncurrent fuel economy benefit (FE_CUR). Current fuel economy benefit(FE_CUR) represents the current fuel economy benefit relative tostoichiometric operation provided by lean operation and is calculatedbased on operating conditions. In particular, as described in U.S.patent application Ser. No. 09/528,217, titled “METHOD AND APPARATUS FORCONTROLLING LEAN-BURN ENGINE BASED UPON PREDICTED PERFORMANCE IMPACT”,filed concurrently with the present application on Mar. 17, 2000,assigned to the same assignee as the present application, which ishereby expressly incorporated by reference, a performance impact is setas a percentage fuel economy benefit/loss associated with engineoperation at a selected lean or rich operating condition relative to areference stoichiometric operating condition at MBT, the controller 12first determines whether the lean-burn feature is enabled. If thelean-burn feature is enabled as, for example indicated by the lean-burnrunning flag LB_RUNNING_FLG being equal to logical one, the controller12 determines a first value TQ_LB representing an indicated torqueoutput for the engine when operating at the selected lean or richoperating condition, based on its selected air-fuel ratio LAMBSE and thedegrees DELTA_SPARK of retard from MBT of its selected ignition timing,and further normalized for fuel flow. Then, controller 12 determines asecond value TQ_STOICH representing an indicated torque output for theengine 10 when operating with a stoichiometric air-fuel ratio at MBT,likewise normalized for fuel flow. In particular, TQ_LB is determined asa function of desired engine torque, engine speed, desired air-fuelratio, and DELTA_SPARK. Further, TQ_STOICH is determined as a functionof desired engine torque and engine speed. Next, the controller 12calculates the lean-burn torque ratio TR_LB by dividing the firstnormalized torque value TQ_LB with the second normalized torque valueTQ_STOICH.

Continuing, the controller 12 determines a value SAVINGS representativeof the cumulative fuel savings to be achieved by operating at theselected lean operating condition relative to the referencestoichiometric operating condition, based upon the air mass value AM,the current (lean or rich) lean-burn air-fuel ratio (LAMBSE) and thedetermined lean-burn torque ratio TR_LB, whereinSAVINGS=SAVINGS+(AM*LAMBSE*14.65*(1−TR)).The controller 12 then determines a value DIST_ACT_CUR representative ofthe actual miles traveled by the vehicle since the start of the lasttrap purge or desulfurization event. While the “current” actual distancevalue DIST_ACT_CUR is determined in any suitable manner, in theexemplary system, the controller 14 determines the current actualdistance value DIST_ACT_CUR by accumulating detected or determinedinstantaneous values VS for vehicle speed.

Because the fuel economy benefit to be obtained using the lean-burnfeature is reduced by the “fuel penalty” of any associated trap purgeevent, in the exemplary system, the controller 12 determines the“current” value FE_CUR for fuel economy benefit only once per NOx fillcycle. And, because the purge event's fuel penalty is directly relatedto the preceding trap “fill,” the current fuel economy benefit valueFE_CUR is preferably determined at the moment that the purge event isdeemed to have just been completed, as described below.

Continuing with FIG. 3, in step 328, maximum fuel economy benefit iscalculated as a function (f₁) of maximum fuel economy benefit andtemporary fuel economy benefit. In this way, the fuel economy benefitprovided by a decontaminated emission control device is filtered overseveral NOx fill/purge cycles. In a preferred embodiment, the filteringis performed by a rolling average function of the form in the followingequation where (fk) is a filter coefficient between zero and one. Thoseskilled in the art will recognize, in view of this disclosure, this as asingle pole low pass filter.output=(1−ƒk)output+(ƒk)input or,output=(1−ƒk)old_output+(ƒk)input, old_output=outputThus, according to the present invention, it is possible to determinethe fuel economy benefit provided by a decontaminated emission controldevice.

Referring now to FIG. 4, a routine is described for determining thepresent, or current, fuel economy benefit that is being provided byoperating lean of stoichiometry with the emission control device 72 inits present state, be it contaminated or decontaminated. First, in step410, a determination is made as to whether a NOx fill/purge cycle hasjust been completed. When the answer to step 410 is YES, the routinecontinues to step 412 where parameter OLD_FE_CUR is set equal toparameter FIL_FE_CUR. Next, in step 414, the routine calculates thecurrent fuel economy benefit (FE_CUR). Next, in step 416, the routinecalculates the filtered current fuel economy benefit (FIL_FE_CUR) basedon a filtered value of the current fuel economy benefit, and parameterOLD_FE_CUR. In other words, the current fuel economy benefit(FIL_FE_CUR) represents the fuel economy benefit that will be realizedif the system continues to operate as it currently does and nodecontamination is performed. Accordingly, (FIL_FE_CUR) is the fueleconomy benefit that will be achieved by not performing adecontamination cycle.

In a preferred embodiment, function (ƒ₂) represents the rolling averagefunction describe above herein. Thus, according to the presentinvention, a fuel economy benefit averaged over several NOx fill/purgecycle can be determined. This value can then be used to advantage invarious ways since it indicates an on-line measure of the improved fueleconomy performance provided by lean operation averaged to remove cycleto cycle variation.

Referring now to FIG. 5, a routine is described for determining a fueleconomy penalty experienced by performing a decontamination cycle. Morespecifically, in a preferred embodiment, a decontamination cycle thatremoves SOx. First, in step 510, a determination is made as to whether adecontamination cycle has just been completed. When the answer to step510 is YES, the routine continues to step 512 where a fuel economypenalty is calculated. The current fuel economy penalty of the lastdecontamination cycle (CUR_FE_PENALTY) is calculated by dividing theexcess fuel used to generate heat or the excess fuel used to operate inone condition compared to another condition by the distance betweendecontamination cycles. In other words, the penalty of performing adecontamination cycle is spread over the distance between twodecontamination cycles. Next, in step 514, a filtered fuel economypenalty is calculated by filtering the current fuel economy penaltyaccording to function (ƒ₃) which, in a preferred embodiment, representsthe rolling average function describe above herein. Thus, according tothe present invention, it is possible to determine the fuel economypenalty experienced by performing a decontamination cycle. In analternative embodiment, the fuel economy penalty to perform adecontamination cycle can be set to a predetermined value.

Those skilled in the art will recognize, in view of this disclosure,various alterations of the present invention that achieve a similarresult. For example, the average excess fuel used during severaldecontamination cycles can be divided by the total distance between allof the decontamination cycles, thereby providing an averaged fueleconomy penalty for performing a decontamination cycle.

In an alternate embodiment, fuel economy penalty to perform adecontamination cycle can be stored as a function of vehicle and/orengine operating parameters. For example, fuel economy penalty can bestored versus vehicle speed and exhaust gas temperature experiencedbefore performing said decontamination cycle. Those skilled in the artwill recognize, in view of this disclosure, various other factors thataffect a fuel economy penalty to perform a decontamination cycle suchas, for example, engine speed, mass air flow, manifold pressure,ignition timing, air-fuel ratio, exhaust gas recirculation amount, andengine torque.

In yet another embodiment, fuel economy penalty can be determined as nowdescribed. First, controller 12 updates a stored value DIST_ACT_DSXrepresenting the actual distance that the vehicle has traveled since thetermination or “end” of the immediately-preceding desulfurization, ordecontamination, event. Then, the controller 12 determines whether adesulfurization event is currently in progress. While any suitablemethod is used for desulfurizing the trap, an exemplary desulfurizationevent is characterized by operation of some of the engine's cylinderswith a lean air-fuel mixture and other of the engine's cylinders with arich air-fuel mixture, thereby generating exhaust gas with aslightly-rich bias. Next, the controller 12 determines the correspondingfuel-normalized torque values TQ_DSX_LEAN and TQ_DSX_RICH, as a functionof current operating conditions. In particular, TQ_DSX_LEAN andTQ_DSX_RICH are determined as functions of desired engine torque, enginespeed, desired air-fuel ratio, and DELTA_SPARK. Then, the controller 12further determines the corresponding fuel-normalized stoichiometrictorque value TQ_STOICH as a function of desired engine torque and enginespeed. The controller 12 then calculates a cumulative fuel economypenalty value, as follows:PENALTY=PENALTY+(AM/2*LAMBSE*14.65*(1−TR _(—)DSX_LEAN))+(AM/2*LAMBSE*14.65*(1−TR _(—) DSX_RICH))Then, the controller 12 sets a fuel economy penalty calculation flag tothereby ensure that the current desulfurization fuel economy penaltymeasure FE_PENALTY_CUR is determined immediately upon termination of theon-going desulfurization event.

If the controller 12 determines that a desulfurization event has justbeen terminated, the controller 12 then determines the current valueFE_PENALTY_CUR for the fuel economy penalty associated with theterminated desulfurization event, calculated as the cumulative fueleconomy penalty value PENALTY divided by the actual distance valueDIST_ACT_DSX. In this way, the fuel economy penalty associated with adesulfurization event is spread over the actual distance that thevehicle has traveled since the immediately-prior desulfurization event.Next, the controller 12 calculates a rolling average value FE_PENALTY ofthe last m current fuel economy penalty values FE_PENALTY_CUR to therebyprovide a relatively-noise-insensitive measure of the fuel economyperformance impact of such desulfurization events. The value FE_PENALTYcan be used in place of value FIL_FE_PENALTY. By way of example only,the average negative performance impact or “penalty” of desulfurizationtypically ranges between about 0.3 percent to about 0.5 percent of theperformance gain achieved through lean-burn operation. Finally, thecontroller 23 resets the fuel economy penalty calculation flagFE_PNLTY_CALC_FLG, along with the previously determined (and summed)actual distance value DIST_ACT_DSX and the current fuel economy penaltyvalue PENALTY, in anticipation for the next desulfurization event.

Referring now to FIG. 6, a routine is described for determining whetherto commence, or begin, a decontamination cycle. First, in step 610, adetermination is made as to whether the maximum potential fuel economybenefit provided a decontaminated emission control device minus thecurrent fuel economy benefit being provided by the decontamination cyclein its present condition is greater than the fuel economy penaltyexperienced by performing a decontamination cycle. In particular, thedifference between parameter FIL_FE_MAX and parameter FIL_FE_CUR iscompared to parameter FIL_FE_PENALTY. When the answer to step 610 isYES, the routine has determined that greater fuel economy can beprovided by performing a decontamination cycle rather than continuingwith operating the engine lean of stoichiometry and performing NOxfill/purge cycles. When the answer to step 610 is NO, the routine hasdetermined that greater fuel economy can be provided by continuingoperation in the present condition. In other words, operating with theemission control device in its present condition provides better fueleconomy than attempting to improve operation of the emission controldevice by performing a decontamination cycle. Next, in step 612, adetermination is made as to whether normalized NOx storage ability(FIL_NOX_STORED) of the emission control device is less than limit valueCl. Normalized stored NOx (FIL_NOX_STORED) is calculated as describedlater herein with particular reference to FIGS. 9 and 10. When theanswer to step 612 is YES, the routine continues to step 613 where adetermination is made as to whether vehicle distance traveled since thelast decontamination cycle is greater than limit distance(DISTANCE_LIMIT). When the answer to step 613 is YES, the routinecontinues to step 614 where a determination is made as to whetherparameter A1 is equal to one. Parameter A1 is determined based onvehicle activity as described later herein with particular reference toFIG. 7. When the answer to step 614 is YES a decontamination cycle isbegun in step 616. The embodiment shown in FIG. 6 is that for theexample of a port fuel injected engine. In an alternate embodiment whichcan be used for direct injection engines, step 614 is eliminated. Thisis because in port fuel injected engines, it is challenging to providewell controlled decontamination temperatures under all operatingconditions. However, in a direct injection engine, since fuel can beinjected during the exhaust stroke to heat the exhaust system,decontamination can be performed at almost any time.

Referring now to FIG. 7, a routine is described for determining vehicleactivity. First, in step 710, a the routine calculates engine power(Pe). In a preferred embodiment, this is the actual engine power,however, in a preferred embodiment, desired engine power can be used.Also, various other parameters can be used in place of engine power,such as, for example: vehicle speed, engine speed, engine torque, wheeltorque, or wheel power. Next, in step 712, engine power (Pe) is filteredwith a high pass filter G₁(s), where s is the Laplace operator known tothose skilled in the art, to produce high pass filtered engine power(HPe). Next, in step 714, the absolute value (AHPe) of the pass filteredengine power (HPe) is calculated. In step 716, the absolute value (AHPe)is low pass filtered with filter G₁(s) to produce signal (LAHPe). Instep 718, adjustment factor K1 calculated as a predetermined function gof signal (LAHPe). Then, in step 720, a determination is made as towhether signal (LAHPe) is less than the calibration parameter(DESOX_VS_ACT_ENABLE_CAL). When the answer to step 720 is YES, parameterA1 is set to one in step 722. Otherwise, value A1 is set to zero in step724.

Referring now to FIG. 8, a graph of function g shows how adjustmentfactor K1 varies as a function of signal (LAHPe) in a preferredembodiment. As shown in the preferred embodiment, as vehicle activityincreases, adjustment factor K1 is reduced. As vehicle activitydecreases, adjustment factor K1 is increased to a maximum value of 0.7.

Referring now to FIGS. 9 and 10, a routine for determining NOx stored inan emission control device is described. In particular, the routinedescribes a method for determining a consistent measure of NOx storedthat can be averaged over several NOx purge/fill cycles. First, in step910, a determination is made as to whether a NOx purge has just beencompleted. In an alternate embodiment, an additional check as to whetherlean operation has commenced can also be used. When the answer to step910 is YES, NOx stored estimated (NOX_STORED) is reset to zero in step912. In particular, the routine assumes that a complete NOx purge wascompleted and all stored NOx was removed. However, in an alternateembodiment, if only part of the NOx was purged, NOx stored in step 912would be set to this partial value rather than zero. Next, in step 913,flag Z is set to zero to indicate that the stored NOx value is not fullyestimated. Next, in step 914, a determination is made as to whether theengine is operating lean of stoichiometry. When the answer to step 914is YES, the routine continues to step 916. In step 916, a calculation offeedgas NOx (NOX_FG) based on operating conditions is generated. Inparticular, feedgas NOx generated by the engine is calculated based onfunction (h1) using operating conditions such as, for example, SIGNAL1(or desired air-fuel ratio of the engine), mass air flow ({dot over (m)}air), engine temperature (TENG), and engine speed (RPM). This feedgasNOx can then be used to represent the NOx entering Nox trap 72. Thoseskilled in the art will recognize, in view of this disclosure, thatvarious additional factors can be used such as factors that account foran NOx storage or reduction due to activity of three way catalyst 70.

Continuing with FIG. 9, in step 918, a determination is made as towhether the ratio of NOx exiting trap 72 to NOx entering trap 72 isgreater than threshold C2. For example, threshold C2 can be set to 0.65.When the answer to step 918 is NO, a NOx difference (NOX_DELTA) iscalculated between NOx entering (NOX_FG) and NOx exiting (SIGNAL2) instep 920. Next, in step 922, an accumulated NOx storage (NOX_STORED) isdetermined by numerically summing NOx difference (NOX_DELTA). When theanswer to step 918 is YES, flag Z is set to one to indicate that aconsistent measure of NOx stored has been completed and fully estimated.

Referring now to FIG. 10, in step 1012, a determination is made as towhether a NOx purge has just been completed. When the answer to step1012 is YES, the routine continues to step 1014. In step 1014, filterednormalized NOX stored (FIL_NOX_STORED) is calculated by filtering NOxstored (NOX_STORED) according to function (ƒ₄) which, in a preferredembodiment, represents the rolling average function describe aboveherein.

Thus, according to the present invention, it is possible to calculate avalue representing a consistant and normalized NOx storage value thatcan be used in determining degradation and determining whether toperform a decontamination cycle.

Referring now to FIG. 11, a routine is described for using first outputsignal (SIGNAL1) of sensor 140 for performing closed loop air-fuel ratiocontrol. First, in step 1110, a determination is made as to whether theabsolute value of the difference between SIGNAL1 and stoichiometricair-fuel ratio (air_fuel_stoich) is greater than a predetermineddifference (D1). In other words, a determination is made as to whetherthe first output signal of exhaust sensor 140 is indicating an exhaustair-fuel ratio away from stoichiometry. When the answer to step 1110 isYES, the routine continues to step 1112. In step 1112, the routinedetermines an air-fuel error (afe) based on the difference betweendesired air-fuel ratio (air_fuel_desired) and the first output signal(SIGNAL1). Next, in step 1114, the routine generates fuel injectionsignal (fpw) based on the determined error (afe) and the cylinder charge(m_cyl_air) and desired air-fuel ratio (air_fuel_desired). In addition,function g2 is used to modify the air-fuel error (afe) and can representvarious control functions such as, for example, a proportional, integraland derivative controller. Also, function g1 is used to convert thedesired mass of fuel entering the cylinder into a signal that can besent to fuel injector 66. Also, those skilled in the art will recognize,in view of this disclosure, that various other corrections involvinginformation from other exhaust gas sensors can be used. For example,additional corrections from sensor 76 can be used.

When the step 1110 is NO, the routine continues to step 1116 andcalculates fuel injection signal (fpw) based on the cylinder chargeamount and the desired air-fuel ratio using function g1. Thus, accordingto the present invention, it is possible to improve open-loop fuelingcontrol using the first output of sensor 140, which is locateddownstream of NOx trap 72, whenever the first output signal indicates avalue away from stoichiometry. In this way, NOx storage and oxygenstorage, as well as NOx reduction, do not adversely closed-loop feedbackair-fuel control using a sensor located downstream of a NOx trap.

Referring now to FIG. 12, an alternate routine to that described in FIG.11 is shown. In this alternate routine, various timers are used to gateout the first output of exhaust sensor 140 for use in feedback air-fuelratio control whenever it is determined that one of the followingconditions is present: oxgen is being stored in NOx trap 72, and/ornitrogen oxide is being released and reduced by a reducing constituentin the exhaust gas in NOx trap 72. Also, this alternate embodiment canbe used to advantage to determine when to enable monitoring of exhaustsensor 140 as described later herein with particular reference to FIGS.13 and 14.

Continuing with FIG. 12, in step 1210, a determination is made as towhether the desired air-fuel ratio (air_fuel_desired) has been changed.In particular, a determination is made as to whether the desiredair-fuel ratio has changed from rich or stoichiometric to lean, orwhether the desired air-fuel ratio has changed from lean tostoichiometric or rich. When the answer to step 1210 is YES, the counterC3 is reset to zero. Otherwise, in step 1214, counter C3 is incremented.Next, in step 1216, a determination is made as to whether the desiredair-fuel ratio is stoichiometric or rich. When the answer to step 1216is YES, a determination is made as to whether counter C3 is greater thanthreshold value D2 in step 1218. Otherwise, when the answer to step 1216is NO, a determination is made as to whether counter C3 is greater thanthreshold value D3 in step 1220. When the answer to either step 1218 orstep 1220 is YES, the routine enables monitoring in step 1222.

In other words, duration D2 and duration D3 represent periods beforewhich first output of exhaust sensor 140 cannot be used for feedbackcontrol because it will indicate stoichiometric even when the exhaustair-fuel ratio entering NOx trap 72 is not stoichiometric. Thus, whenchanging from stoichiometric or rich to lean, first output of exhaustsensor 140 is valid for monitoring or feedback control after durationD3. Similarly, when changing from lean operation to rich orstoichiometric operation, first output of exhaust sensor 140 is validfor monitoring or feedback control after duration D2. In a preferredembodiment, duration D2 is based on oxygen storage of trap 72 andduration D3 is based on both oxygen storage and NOx storage of trap 72.Stated another way, once the oxygen storage is saturated when changingfrom rich to lean, SIGNAL1 is indicative of the air-fuel ratio enteringtrap 72. And once the oxygen stored and NOx stored is reduced whenchanging from lean to rich, SIGNAL1 is indicative of the air-fuel ratioentering trap 72.

Continuing with FIG. 12, in step 1224, a determination of air-fuel error(afe) is made by subtracting desired air-fuel ratio (air_fuel_desired)and first output of exhaust sensor 140 (SIGNAL1). Next, in step 1226,fuel injection signal (fpw) is calculated in a manner similar to step1114.

When the answers to either step 1218 or step 1220 are NO, the routinecontinues to step 1228 to calculate fuel injection signal (fpw) asdescribed herein in step 1116. Thus, according to the present invention,it is possible to utilize the first output of exhaust sensor 140 forfeedback air-fuel control.

Referring now to FIG. 13, a routine is described for determiningdegradation of the second output signal of exhaust sensor 140. Inparticular, a routine is described for determining degradation ofindicated NOx concentration based on the first output signal of exhaustgas sensor 140, when the first output signal is indicative of an exhaustair-fuel ratio. First, in step 1310, a determination is made as towhether monitoring is enabled as described in step 1222, or whether theengine is operating in a near stoichiometric condition. Further, adetermination is also made as to whether the first output signal ofexhaust sensor 140 is degraded. In other words, when SIGNAL1 isindicative of the air-fuel ratio entering trap 72, it can be used toprovide an estimate of NOx concentration exiting trap 72. When theanswer to step 1310 is YES, the routine continues to step 1312. In step1312, the routine estimates the second output signal (est_signal 2)based on several conditions. In particular, function h2 is used with thefeed gas NOx (NOx_fg) and the first output signal of exhaust sensor 140(SIGNAL1). In other words, the routine attempts to estimate NOx exitingtrap 72 based on NOx entering trap 72 and exhaust air-fuel ratio. Inaddition, various other dynamic effects of NOx trap 72 can be added toaccount for oxygen storage and nitrogen oxide and oxygen reduction.Further, efficiency of trap 72 can be included to estimate NOx exitingbased on NOx entering trap 72. However, if performed duringstoichiometric operation, it can be assumed that net NOx stored isconstant. Next, in step 1314, the absolute value of the differencebetween the estimated NOx exiting trap 72 (EST_SIGNAL2) and measuredsecond output of exhaust sensor 140 (SIGNAL2) is compared to thresholdvalue C4. When the answer to step 1314 is YES, counter C5 is incrementedin step 1316. Next, in step 1318, a determination is made as to whethercounter C5 is greater than threshold C6. When the answer to step 1318 isYES, the routine indicates degradation of the second output of exhaustsensor 140 in step 1320.

Thus, according to the present invention, it is possible to determinewhen the NOx sensor, which is the second output of exhaust sensor 140,has degraded by comparing to an estimated value of exiting NOx trap 72.

Referring now to FIG. 14, a routine is described for determiningdegradation of the second output signal of sensor 140 based on the firstoutput signal of sensor 140. First, a determination is made in step 1410as to whether monitoring has been enabled or whether operating nearstoichiometry. When the answer to step 1410 is YES, the routinecontinues to step 1412. In step 1412, the routine estimates air-fuelratio that should be measured by the first output signal (SIGNAL1) ofexhaust sensor 140. In other words, the routine estimates exhaustair-fuel ratio exiting NOx trap 72 based on various operatingparameters. The estimated air-fuel ratio (AFTP_EST) is estimated basedon air-fuel ratio measured by sensor 76 (UEGO), mass airflow measured bymass airflow sensor 100, and fuel injection amount (fpw) in a preferredembodiment. Those skilled in the art will recognize, in view of thisdisclosure, various other signals and methods that can be used toestimate exhaust air-fuel ratio exiting a NOx trap. For example, dynamiceffects of both catalyst 70 and 72 can be included that account for NOxstorage, oxygen storage, temperature effects, and various other effectsknown to those skilled in the art.

Continuing with FIG. 14, in step 1414, the absolute value of thedifference between the estimated exhaust air-fuel ratio (AFTP_EST) inthe first output signal of exhaust gas sensor 140 (SIGNAL1) is comparedto threshold C7. When the answer to step 1414 is YES, counter C8 isincremented in step 1416. Next, in step 1418, counter C8 is compared tothreshold C9 in step 1418. When the answer to step 1418 is YES, anindication is provided in step 1420 that both the first output signaland a second output signal of exhaust sensor 140 have been degraded.Thus, according to the present invention, it is possible to determinethat the NOx concentration measured by the second output signal ofexhaust sensor 140 is degraded when it is determined that the oxygenpartial pressure indicated in the first output signal of exhaust sensor140 has been degraded.

Referring now to FIGS. 15A–15C, these figures show an example ofoperation according to the present invention. In particular, the graphsshow when first output signal (SIGNAL1) of sensor 140 is valid forair-fuel control or for monitoring. FIG. 15A shows air-fuel ratioentering NOx trap 72 versus time. FIG. 15B shows air-fuel ratio exitingNOx trap 72 versus time. FIG. 15C indicates whether first output signal(SIGNAL1) of sensor 140 is valid for air-fuel control or for monitoring.

Before time t1, the entering air-fuel ratio and exiting air-fuel ratioare both lean and first output signal (SIGNAL1) is valid for control ormonitoring. Then, at time t1, a determination is made to end leanoperation and purge NOx stored in trap 72 due to tailpipe grams ofNOx/mile, or because a fuel economy benefit is no longer provided byoperating lean, or for various other reasons as described above herein.At time t1, entering air-fuel ratio is changed from lean to rich.Similarly, at time t1, air-fuel ratio exiting changes to stoichiometricuntil all stored NOx and oxygen are reduced, which occurs at time t2.Thus, according to the present invention, the stoichiometric air-fuelratio measured downstream of NOx trap 72 during the interval from timet1 to time t2, is not equal to the air-fuel ratio upstream of NOx trap72. After time t2, a rich exhaust air-fuel ratio is measured downstreamof NOx trap 72 and this measurement can be used for air-fuel control ormonitoring. At time t3, entering air-fuel is changed back to a leanair-fuel ratio. Again, air-fuel ratio exiting changes to stoichiometricuntil all the oxygen storage capacity of NOx trap 72 is saturated attime t4. Thus, according to the present invention, the stoichiometricair-fuel ratio measured downstream of NOx trap 72 during the intervalfrom time t3 to time t4 is not equal to the air-fuel ratio upstream ofNOx trap 72. After time t4, the entering air-fuel ratio can be measuredby sensor 140 and thus can be used for control or monitoring.

Those skilled in the art will recognize in view of this disclosure thatthe above methods are applicable with any decontamination method. In apreferred embodiment, the decontamination method described in U.S. Pat.No. 5,758,493, which is hereby incorporated by reference, can be used.

Although several examples of embodiments which practice the inventionhave been described herein, there are numerous other examples whichcould also be described. The invention is therefore to be defined onlyin accordance with the following claims.

1. A method for controlling an internal combustion engine of a vehicle,the engine coupled to an emission control device susceptible toreversible sulfur contamination, the method comprising: generating anindication of vehicle activity based on an operating condition;determining a performance impact of performing a decontamination cycle,where said decontamination cycle reverses the reversible sulfurcontamination and said performance impact is a fuel economy penaltyrelative to near stoichiometric operation, wherein said nearstoichiometric operation includes oscillatory control aboutstoichiometry; and operating engine based on said indication and saidperformance impact, where said engine operation includes said nearstoichiometric operation.
 2. The method recited in claim 1 wherein saidoperating the engine further comprises enabling decontamination cyclesbased on said indication and said performance.
 3. The method recited inclaim 1 wherein said operating the engine further comprises enablingdecontamination cycles based on said indication and said performance. 4.The method recited in claim 1 wherein said operating the engine furthercomprises operating the engine to reduce reversible contamination whensaid performance impact is less than a predetermined value, wherein saidcontamination includes sulfur contamination.
 5. The method recited inclaim 4 wherein said predetermined value is a performance benefit ofperforming the decontamination.
 6. The method recited in claim 1 whereinsaid operating the engine further comprises operating the engine toremove reversible contamination when said performance impact is lessthan a predetermined value.
 7. The method recited in claim 6 whereinsaid predetermined value is a performance benefit of performing thedecontamination.
 8. A method for controlling an internal combustionengine of a vehicle, the engine coupled to an emission control devicesusceptible to reversible sulfur contamination, the method comprising:generating an indication of vehicle activity based on operatingconditions; determining a performance impact of operating the enginewith a contaminated emission control device, said performance impact isa fuel economy penalty relative to near stoichiometric operation,wherein said near stoichiometric operation includes oscillatory controlabout stoichiometry; and operating engine based on said indication andsaid performance impact.
 9. The method recited in claim 8 wherein saidoperating the engine further comprises enabling decontamination cyclesbased on said indication and said performance.
 10. The method recited inclaim 8 wherein said operating the engine further comprises operatingthe engine to remove reversible contamination when said performanceimpact is less than a predetermined value.
 11. The method recited inclaim 10 wherein said predetermined value is a performance benefit ofperforming the decontamination.
 12. A method for controlling an internalcombustion engine of a vehicle, the engine coupled to an emissioncontrol device susceptible to sulfur contamination, the methodcomprising: generating an indication of vehicle activity based on anoperating condition; determining a performance impact of performing asulfur decontamination cycle, where said sulfur decontamination cyclereduces the sulfur contamination; and operating the engine lean to storeNOx in the emission control device, and then operating the engine nearstoichiometric or rich to purge said stored NOx; and during a firstvehicle activity and performance impact condition, initiating saiddecontamination cycle and enabling lean operation, and during a secondvehicle activity and performance impact condition, operating the engineat near stoichiometric, wherein said near stoichiometric operationincludes oscillatory control about stoichiometry.
 13. A method forcontrolling an internal combustion engine of a vehicle, the enginecoupled to an emission control device susceptible to sulfurcontamination, the method comprising: generating an indication ofvehicle activity based on an operating condition, said operatingcondition being at least one parameter selected from the groupconsisting of engine power, vehicle speed, engine speed, engine torque,wheel torque, and wheel power; determining a performance impact ofperforming a sulfur decontamination cycle, where said sulfurdecontamination cycle reduces the sulfur contamination; and operatingthe engine based on said indication and said performance impact, whereinduring a first vehicle activity and performance impact condition,initiating said decontamination cycle and enabling lean operation, andduring a second vehicle activity and performance impact condition,operating the engine at near stoichiometric, wherein said nearstoichiometric operation includes oscillatory control aboutstoichiometry.
 14. A system for an internal combustion engine of avehicle, the engine coupled to an emission control device susceptible toreversible sulfur contamination, the system comprising: a sensor coupleddownstream of the emission control device, said sensor providing a firstand second signal, said first signal indicative of exhaust air-fuelratio and said second signal indicative of nitrogen oxides in theexhaust gas; and a controller for generating an indication of vehicleactivity based on an operating condition; determining a performanceimpact of performing a sulfur decontamination cycle, where said sulfurdecontamination cycle reverses the reversible sulfur contamination; andoperating the engine based on said indication, said performance impact,said first signal, and said second signal, wherein said operating theengine includes during a first vehicle activity and performance impactcondition, initiating said decontamination cycle and enabling leanoperation, and during a second vehicle activity and performance impactcondition, operating the engine at near stoichiometric, wherein saidnear stoichiometric operation includes oscillatory control aboutstoichiometry.